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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
class VarInt
{
public:
static UInt32 ReadUnsigned(PTR_UInt8 & pbEncoding)
{
UIntNative lengthBits = *pbEncoding & 0x0F;
size_t negLength = s_negLengthTab[lengthBits];
UIntNative shift = s_shiftTab[lengthBits];
UInt32 result = *(PTR_UInt32)(pbEncoding - negLength - 4);
result >>= shift;
pbEncoding -= negLength;
return result;
}
//
// WARNING: This method returns the negative of the length of the value that it just skipped!
//
// This was helpful in the GC info scan loop because it allowed us to always skip past unsigned values in
// the body of the loop. At the end of loop, we use this negative sign to distinguish between two cases
// and that allows us to decode the unsigned value that we need outside of the loop. Note that we encode
// the negatives in the s_negLengthTable to avoid any additional operations in the body of the GC scan
// loop.
//
static IntNative SkipUnsigned(PTR_UInt8 & pbEncoding)
{
UIntNative lengthBits = *pbEncoding & 0x0F;
size_t negLength = s_negLengthTab[lengthBits];
pbEncoding -= negLength;
return negLength;
}
static UIntNative WriteUnsigned(PTR_UInt8 pbDest, UInt32 value)
{
if (pbDest == NULL)
{
if (value < 128)
return 1;
if (value < 128*128)
return 2;
if (value < 128*128*128)
return 3;
if (value < 128*128*128*128)
return 4;
return 5;
}
if (value < 128)
{
*pbDest++ = (UInt8)(value*2 + 0);
return 1;
}
if (value < 128*128)
{
*pbDest++ = (UInt8)(value*4 + 1);
*pbDest++ = (UInt8)(value >> 6);
return 2;
}
if (value < 128*128*128)
{
*pbDest++ = (UInt8)(value*8 + 3);
*pbDest++ = (UInt8)(value >> 5);
*pbDest++ = (UInt8)(value >> 13);
return 3;
}
if (value < 128*128*128*128)
{
*pbDest++ = (UInt8)(value*16 + 7);
*pbDest++ = (UInt8)(value >> 4);
*pbDest++ = (UInt8)(value >> 12);
*pbDest++ = (UInt8)(value >> 20);
return 4;
}
*pbDest++ = 15;
*pbDest++ = (UInt8)value;
*pbDest++ = (UInt8)(value >> 8);
*pbDest++ = (UInt8)(value >> 16);
*pbDest++ = (UInt8)(value >> 24);
return 5;
}
private:
static Int8 s_negLengthTab[16];
static UInt8 s_shiftTab[16];
};
__declspec(selectany)
Int8 VarInt::s_negLengthTab[16] =
{
-1, // 0
-2, // 1
-1, // 2
-3, // 3
-1, // 4
-2, // 5
-1, // 6
-4, // 7
-1, // 8
-2, // 9
-1, // 10
-3, // 11
-1, // 12
-2, // 13
-1, // 14
-5, // 15
};
__declspec(selectany)
UInt8 VarInt::s_shiftTab[16] =
{
32-7*1, // 0
32-7*2, // 1
32-7*1, // 2
32-7*3, // 3
32-7*1, // 4
32-7*2, // 5
32-7*1, // 6
32-7*4, // 7
32-7*1, // 8
32-7*2, // 9
32-7*1, // 10
32-7*3, // 11
32-7*1, // 12
32-7*2, // 13
32-7*1, // 14
0, // 15
};
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